1. Field of the Invention
The present invention relates to a technique to protect information stored in a semiconductor memory.
2. Description of the Background Art
Semiconductor memories are widely used to store contents for mobile phones, PDAs (personal digital assistants), game machines or the like. If memories incapable of rewriting data (such as mask ROMs or the like) are used as semiconductor memories, contents stored therein can not be unauthorizedly rewritten. Recently, however, flash memories capable of rewriting data are widely used for data recording in mobile phones, personal computers or the like. Flash memories of higher capacity and lower cost have become widespread and have grown in demand as highly convenient data recording media taking the place of magnetic memory media.
As discussed above, flash memories capable of rewriting data are suitable for and widely used for recording and storing multimedia data. Further, being convenient, the flash memories are considered to be used to store contents for mobile phones, PDAs, game machines or the like.
Since the flash memories have the property of rewriting data, however, it is necessary to protect the stored contents against tampering. Japanese Patent Application Laid Open Gazette No. 2005-108273 discloses a technique to protect rewritable semiconductor memories such as NAND flash memories. The semiconductor memory shown in this document has a construction in which a protect flag is set to some of memory blocks and only if the protect flag indicates a predetermined value, data can be written thereto.
The technique disclosed in Japanese Patent Application Laid Open Gazette No. 2005-108273 is mainly intended for data mis-erase protection. In other words, this is a technique to provide protection against mistakenly erasing of data with a wrong command. Therefore, since data can be written and erased by controlling the protect flag, it is impossible to prevent tampering of data by vicious persons. On the other hand, if the protect flag can not be controlled, it becomes impossible to write or erase data to/from the block and this causes inconvenience.
Further, some semiconductor memories are each provided with a write protect terminal. This technique is intended to provide protection for the entire memory cell and also causes inconvenience.